The platinum catalysts deposited on silicon nitride (Si3N4) were studied in the reaction of the deep oxida-tion of methane. The surface properties of Pt/Si3N4 samples were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy before and after reaction. It was established that the average particle size of platinum for the fresh catalysts was equal to 1.7-5.3 nm, while the formation of Pt crystallites with the size 30-70 nm was found after catalytic reaction. It was supposed that deactivation of platinum catalysts in the deep oxidation of methane is connected with crystallization of metallic particles and their entrainment with the reaction products during catalysis.